This invention relates to non-complimentary or single-ended noise reduction systems and more particularly concerns single-ended noise reduction systems used in conjunction with other types of signal processors to form a composite signal processor and single-ended noise reduction system.
Single-ended noise reduction systems such as are described in my U.S. Pat. Nos. 4,647,867 and 4,696,044 are now quite well known. Composite systems combining some form of signal processor with single ended noise reduction systems are also known, such as Rocktron Corporation's Model 300A Compressor and Model RX20 Audio Enhancer. These composite systems, and other such systems presently in use, utilize a prior art configuration in which the signal processor circuit is followed by the single-ended noise reduction system circuitry. It is desirable to have the noise reduction system following the signal processing circuit so as to reduce both the noise present in the input signal and the noise inherent in the signal processor. However, there are undesirable limitations in such systems as they are presently practiced.
Typically, these composite systems are designed to use the composite circuits together or to use either circuit separately with the other circuit being bypassed. Consequently, the noise reduction system of the composite system must be able to handle the full dynamic range of the unaltered input signal which typically could be as small as 50dB. This presents a problem when a compressor circuit precedes the noise reduction system and reduces the dynamic range of the input signal. Since compressor circuits allow the user to change the dynamic range of the input signal, it is, as a practical matter, impossible for the noise reduction circuit to work in conjunction with the dynamic range that the compressor provides. Because the amount of compression and the ratio of compression are both user adjustable, there is no simple and economical noise reduction circuit which provides the two modes of operation needed to meet the noise reduction requirements of both the unaltered input signal and the compressed signal.
The use of a composite audio enhancer with a single-ended noise reduction system also presents problems. While the detection circuit for the single-ended noise reduction system is optimized to work independently, breathing and pumping are encountered when the detection circuit of the single-ended noise reduction system senses the output of the signal processing device. This occurs because the dynamic filter control signal is itself derived from the enhanced signal which is already dynamically emphasized in relation to its high frequency content, thus introducing more high frequency signal to the filter than was present in the input signal.
It is, therefore, an object of the present invention to provide a composite signal processor and single-ended noise reduction system employing a signal processor circuit preceding a single-ended noise reduction system to provide noise reduction of both the input signal and the signal processor wherein the detection circuit of the single-ended noise reduction system senses the input signal before it is processed. Yet another object of the present invention is to provide a composite signal processor and single-ended noise reduction system which eliminates mistracking of the noise reduction circuit. It is also an object of the present invention to provide a system which permits two modes of operation, either with or without the signal processor in the audio path, without any adverse dynamic effect of the single-ended noise reduction system.